After hundreds of clinical trials, about thirty monoclonal antibodies have been approved by the FDA to-date for treating a variety of indications including cancer, autoimmune disease and infectious agents. One reason that more antibodies have not been approved is that mechanisms of action provided by an antibody alone, such as effector function or blocking receptor-ligand interactions may not be sufficiently powerful to have a substantial therapeutic effect. Antibody-drug conjugates (ADCs) provide additional mechanisms, particularly delivery of a toxic moiety coupled to the antibody to the interior of a cell, thereby killing the cell or otherwise inhibiting its proliferation. Currently two ADCs are marketed: brentuximab vedotin and trastuzumab emtansine. Many other ADCs are at various stages of development. Production of ADC's involves antibody expression and purification, followed by chemical conjugation of the antibody to a drug usually via a linker.